Interconnect cables transport low level voltage signals from a source such as a microphone or a tape or record player to a load such as a pre-amplifier or a power amplifier. The cable must be able to transfer the signal without changing its amplitude and shape within a frequency band covering at least the audible range from 20 through 20,000 Hertz. However, recent advances in amplifier and loudspeaker technology have demonstrated audible benefits if cables are able to transfer signals at frequencies beyond the audible range without distortion such as high frequency roll-off and frequency dependent phase shift.
U.S. Pat. No. 5,393,933 describes an audio loudspeaker cable which achieves an extremely low series inductance combined with a high relative capacitance to obtain a characteristic impedance on the order of magnitude of 2-10 ohms. The effect of this is an almost total absence of roll-off and ringing caused by repeated signal reflections. In addition, the cable has a low loop resistance in order to support a powerful, undistorted bass and lower mid range. According to the patent, these characteristics are achieved by means of a sandwich construction of two relatively heavy, wide conductors separated by a thin layer of dielectric material. Because of the required low loop resistance, this cable is basically a power cable which is able to transmit signals of several amperes without excessive power loss.
Since the signal strength in an audio interconnect cable rarely exceeds a few milliamperes, such cables do not require the heavy conductors used in loudspeaker cables, but, on the other hand, become more sensitive to electromagnetic interference such as hum, noise, cross-talk, and air borne radio frequency signals. Basic protection from interference includes optimizing the geometry of signal carrying conductors by arranging them co-axially or as twisted pairs, and, in addition, individual conductors or groups of conductors are often surrounded by shielding which is connected to ground or to the chassis of interconnected components.
Besides interference from the outside, the weak signals traveling in interconnect cables may be affected by noise generated in the cable itself caused by interaction between wire strands, so called microphony, and by tribo-electric charges and discharges in the dielectric materials which separate the signal carriers from one another and their shielding. Tribo-electric charges may be generated by even minute movement or compression of cables or by ground loops and, in addition, charge build up may be caused by the signals traveling in the cables.
In order to improve the noise to signal ratio, often balanced constructions are preferred over single ended cables, either by replacing the single center conductor in a coaxial cable with a twisted pair or by arranging the second signal carrier as a braided tube concentrically surrounding the center carrier, the whole construction again surrounded by a braided, grounded shield. The relative capacitance and series inductance of such cables, and thus their characteristic impedance, are dependent on dimensions, geometry, and the characteristics of the dielectric and all of these affect cable performance.
The term "strip line" describes a kind of transmission line which is similar in many respects to a coaxial cable. It may, in fact, be regarded as a simple variation of the coaxial line, although it has a great many practical advantages over the latter. The basic strip line is uniform in cross section and consists of a thin center conductor of rectangular cross section situated between two parallel conductors of the same width or wider than the center conductor and serving as ground planes. The three layers in turn are insulated from one another by means of layers of a thin dielectric sheet. When energized, the field is largely confined to the region between the three parallel conductors with the field strength decreasing rapidly away from the strip at a rate relative to the distance between the plates. Thus, with conductors consisting of thin foil and separated by an even thinner dielectric, the outside field is close to zero and, by the same token, such a strip line is closed to a high degree towards interference from signals traveling in nearby lines and airborne RF signals.
In an audio system, interference may be directly audible or may enter amplifiers through feedback loops from either the input or output terminals. Thus, interfering radio frequency signals may audibly change the bias of amplifier stages and may result in indirect distortion of the output signal, and in some cases may cause overheating due to inaudible amplification of high frequency interference. This is the reason why good interconnect cables must be screened from or otherwise rendered insensitive to external magnetic field and RF signals.
A further limitation of flat cables is that, due to the solid conductors employed, the flat cables can only be bent and coiled perpendicularly to the axis of bending and this can constitute a problem when the cables are used as, e.g., microphone or instrument cables.
Accordingly, it is a principal object of the present invention to provide an interconnect cable that eliminates or considerably reduces signal distortion, noise, and interference from surrounding electric and magnetic fields.
It is a further object of the invention to provide such an interconnect cable that is able to transfer signals at frequencies beyond the audile range without distortion such as high frequency roll-off and frequency dependent phase shift.
It is another object of the invention to provide such an interconnect cable that is relatively insensitive to electromagnetic interference such as hum, noise, cross-talk, and air borne radio frequency signals.
An additional object of the invention is to provide such an interconnect cable that is relatively unaffected by noise generated in the cable itself.
Yet a further object of the invention is to provide such an interconnect cable that is insensitive to surrounding magnetic fields and radio frequency signals.
Yet another object of the invention is to provide such an interconnect cable that can be bent and coiled other than perpendicularly to the axis of bending.
Other objects of the present invention, as well as particular features, elements, and advantages thereof, will be elucidated in, or be apparent from, the following description and the accompanying drawing figures.